[PATCH] mm: memfd_luo: account restored large folios correctly

[PATCH] mm: memfd_luo: account restored large folios correctly

[DaeMyung Kang]

memfd_luo_retrieve_folios() always passes 1 to
shmem_inode_acct_blocks() and shmem_recalc_inode(), even when the
restored folio spans multiple pages.

That mismatches the normal shmem path: shmem_add_to_page_cache()
accounts the folio in mapping->nrpages using folio_nr_pages(folio),
and shmem_alloc_and_add_folio() passes the same page count to
shmem_inode_acct_blocks() and shmem_recalc_inode().

As a result, restored large folios are under-accounted in shmem inode
and block accounting.

Use folio_nr_pages(folio) here as well.

Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")
Signed-off-by: DaeMyung Kang <charsyam@gmail.com>
---
 mm/memfd_luo.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
index b8edb9f981d7..6dd0284b90c6 100644
--- a/mm/memfd_luo.c
+++ b/mm/memfd_luo.c
@@ -402,6 +402,7 @@ static int memfd_luo_retrieve_folios(struct file *file,
 		const struct memfd_luo_folio_ser *pfolio = &folios_ser[i];
 		phys_addr_t phys;
 		u64 index;
+		long pages;
 		int flags;
 
 		if (!pfolio->pfn)
@@ -441,14 +442,16 @@ static int memfd_luo_retrieve_folios(struct file *file,
 		if (flags & MEMFD_LUO_FOLIO_DIRTY)
 			folio_mark_dirty(folio);
 
-		err = shmem_inode_acct_blocks(inode, 1);
+		pages = folio_nr_pages(folio);
+
+		err = shmem_inode_acct_blocks(inode, pages);
 		if (err) {
 			pr_err("shmem: failed to account folio index %ld: %d\n",
 			       i, err);
 			goto unlock_folio;
 		}
 
-		shmem_recalc_inode(inode, 1, 0);
+		shmem_recalc_inode(inode, pages, 0);
 		folio_add_lru(folio);
 		folio_unlock(folio);
 		folio_put(folio);
-- 
2.43.0

Re: [PATCH] mm: memfd_luo: account restored large folios correctly

[Pratyush Yadav]

Hi DaeMyung,

On Tue, Apr 14 2026, DaeMyung Kang wrote:

> memfd_luo_retrieve_folios() always passes 1 to
> shmem_inode_acct_blocks() and shmem_recalc_inode(), even when the
> restored folio spans multiple pages.
>
> That mismatches the normal shmem path: shmem_add_to_page_cache()
> accounts the folio in mapping->nrpages using folio_nr_pages(folio),
> and shmem_alloc_and_add_folio() passes the same page count to
> shmem_inode_acct_blocks() and shmem_recalc_inode().
>
> As a result, restored large folios are under-accounted in shmem inode
> and block accounting.
>
> Use folio_nr_pages(folio) here as well.
>
> Fixes: b3749f174d68 ("mm: memfd_luo: allow preserving memfd")
> Signed-off-by: DaeMyung Kang <charsyam@gmail.com>

Thanks for the patch. This bug is already fixed by this patch [0].

[0] https://lore.kernel.org/linux-mm/20260326084727.118437-2-duanchenghao@kylinos.cn/

[...]

-- 
Regards,
Pratyush Yadav